In conventional three speed internal hub transmissions used on bicycles, the load path and therefore the gear ratio is controlled by the position of an element called the "clutch wheel" or "claw clutch". The clutch wheel moves axially along the axle between three axial positions. In conventional designs which are currently in use in the marketplace, the axial position of the clutch wheel is controlled in one direction by a pull chain, cable, or pushrod/bell crank mechanism and in the other direction by a return spring.
When the transmission is transmitting a load, the splines at each end of the clutch wheel are loaded circumferentially and the resulting friction prevents easy axial movement. In order to make the transmission shift under more than zero load, the preload in the return spring is increased. In order to make the transmission shift under more load than this, the preload must be increased proportionally to the load through the transmission. This increase in preload of the return spring must ultimately be overcome by effort at the hand actuator on the handlebar. The preload needed to make the conventional design shift under load makes this effort excessive.
It is therefore desirable to find another source of axial force or forces to move the clutch wheel in both the up and down shifting directions. Ideally it would also be desirable to find a source of axial force that is always proportional to the load through the transmission and is therefore always great enough to overcome the frictional forces that oppose this movement.
European Patent Application 876953 discloses a mechanism which uses a servo principle to move the clutch wheel. As shown in FIGS. 1-4, this mechanism upshifts in a conventional manner using no servo effect to "force" the upshift. Movement of the clutch wheel in the upshifting direction is effected by a displacement of the control cable 73b toward the handlebar actuator. This rotates a bellcrank 71 which pushes on pushrod assembly 69 and 68 (FIG. 2). A preloaded spring 60 is therefore compressed below its installed length and transfers the force to control element assembly 66 and 49. The axial force is then transferred to a clutch wheel 45 by means of a snap ring 63 (FIG. 3). The force to move the clutch wheel 45 in the upshifting axial direction is the result of the energy put into the hand actuator on the handlebar minus the inefficiencies of all the interactions between the handlebar actuator and the clutch wheel. The control elements 69, 68, 60, 66 and 49 must move axially to displace the clutch wheel 45 the same axial distance.
The mechanism downshifts in a servo manner. To initiate a downshift, the cable 73b and the bell crank 71 release the control elements 69, 68, 60, 66, and 49 so that another preloaded spring 61 can apply an unopposed force on an element 49. When the cam lobes on the inside diameter of the clutch wheel permit it, element 49 moves into a valley 47a between the cam lobes. Element 49 simultaneously slides along helical slot 21b in the axle. The angles between the helical slot 21 in the axle and the helical cam inside the clutch wheel cooperate in such a way that the control element 49 becomes axially fixed and therefore the rotation of the clutch wheel is converted to axial displacement as control element 49 slides up helical cam ramp 47c. As in the case of the upshifting sequence, elements 69, 68, 60, 66, and 49 move axially to accomplish a displacement of the clutch wheel the same axial distance.
One feature that distinguishes the present invention from the device disclosed in EP 876953 is that the helical camming servo effect works in one direction only in the aforementioned prior art. Furthermore, in this prior art construction, to be fully enabled to both up and down shift, the device must use both the helical camming servo effect and the conventional simple non-servo pushing method.
The use of an axially moving control element also has certain drawbacks. In a three-speed hub, it must protrude out of the end of the axle at least as much as the combined stroke of two shifts. This is a very vulnerable place to put a delicate, protruding control element. It is also awkward to convert cable displacement into control element displacement because of the 90-degree difference in orientation. The axially moving control element is also awkward to control with a gear motor since a gear motor in its simplest form is most suitable to deliver rotation, not sliding, axial movement. Also, as a general engineering principle, rotation is preferable to sliding because it is less susceptible to cocking and jamming.